The invention relates to a gas guide element with sound-absorbent walls for blocking and damping noise spreading from it into main conduits, which element is made up of a flexible inner hose with a metal element, an outer hose, and a soft and likewise flexible intermediate layer of material disposed between the hose.
In a known gas guide element of this kind according to German Patent Publication DE-OS 27 49 665, the inner hose is comprised of a wide-mesh cloth which is provided on its inner side with a spiral wire as a metal element to reinforce it. The outer hose is constituted by a foil, wherein a foamed material is inserted between this foil and the cloth as a intermediate layer of material. The annular end regions of this gas guide element, which is embodied as flexible and linear as well, are closed by glued-on end caps. According to the ideas of the inventor of this gas guide element, the sound is meant to penetrate the mesh of the cloth of the inner hose, into the intermediate layer of material, which is comprised of a foamed material body, and be damped there. Because of the foamed material body, gas guide elements of this kind are neither temperature- nor abrasion-resistant and for this reason are only partially suited for sound blocking and damping. This kind of gas guide elements does not come into consideration for blocking or damping sound emissions of high sound intensity in hot gasses from gas turbines.
In another gas guide element according to German Patent Publication DE 38 30 346 C2, the inner hose, is embodied of a base element, which is both flexible and perforated, on whose exterior an open-pored foam of polymer material is deposited as a sound blocking material, which is encompassed on its outer circumference face by a compact skin as an outer hose. This gas guide element also does not come into consideration for guiding hot gasses and for damping sound intensities connected with them which occur in gas turbines. Also, the relative reductions of the sound pressure level mentioned in these prior publications allow the conclusion that there is only mediocre sound damping, although considered in the absolute, a reduction of the sound pressure level by 3 dB(A) already signifies a reduction of the sound intensity by half.
Furthermore, German Patent Publication DE 32 20 023 C3 and the identical European Patent Publication EP 0 095 582 disclose a gas guide element whose outer layer is comprised of a mainly closed-celled, elastic, thermoplastic foamed material, whereas its inner layer, which is predominantly intended to take on the task of sound blocking, is embodied of an open-celled, soft foamed material, for example a polyurethane soft foam. The two layers are connected either by means of flame lamination or a bonding agent. An intermediate layer of material between the layers in the form of a reinforcing layer is also considered, for example a random fiber structure, cloth or knit cloth of textile and/or glass fibers. This gas guide element also is considered as a sound damper for hot gasses because of its material compositions.
In gas turbines or air driver pumps to be serviced in large-capacity airplanes, for example in those of Boeing aircraft of the type 747; a sound intensity at the turbine exit of 150 dB(A) and a gas temperature of approximately 200.degree. C. must be expected, and at the main turbines, a sound intensity of 165 dB(A) and a core jet temperature of 820.degree. C. must be expected. This kind of sound intensity signifies a threat to human life. The graphs of this prior publication also disclose little regarding the sound intensity reduction in this range.
And finally, U.S. Pat. No. 3,374,856 discloses a gas guide element of another generic type, which is comprised of a corrugated outer hose of rubber-impregnated or plastic-impregnated woven cloth, while the inner hose is made of a synthetic plastic, e.g. urethane caoutchouc. There is no layer of material as an intermediate covering. FIG. 2 of this patent document shows a bend of a gas guide element of this kind by 180.degree. in order to verify its flexibility. In this prior publication, though, an installation position in this bent form is not even mentioned once in passing. Also, this gas guide element is not suited to blocking or damping sound intensities in hot gasses from gas turbines because of its temperature-sensitive materials.
In addition to these gas guide elements with sound-absorbent walls, there are also a number of sound dampers with nonabsorbing walls, for example according to U.S. Pat. Nos. 2,886,121, 3,187,835, 3,415,337, 3,227,240, European Patent Publications EP 0 346 551 B1, EP 0.493 161 A1, and German Patent Publication DE-OS 2 264 354, which are not the subject of the instant invention.